Julien Michels

Structural Strengthening with Prestressed CFRP Strips with Gradient Anchorage

AbstractThis paper presents the principle and the application of an innovative anchorage technique for prestressed carbon fiber–reinforced polymer (CFRP) strips in structural strengthening. Additionally, large-scale static loading tests of retrofitted concrete beams are shown. The gradient anchorage, based on the adhesive’s ability to undergo accelerated curing at high temperatures, consists of a purely concrete-adhesive strip connection without any mechanical devices, such as bolts or plates. In a first step, this study summarizes anchorage techniques presented in the literature and introduces the basic principles of the new method as well as the necessary components. In a second step, an application on a full-scale RC beam is explained in detail. A commercially-available CFRP strip is prestressed up to 0.6% prestrain and subsequently anchored by sequential epoxy-curing and force-releasing steps at both strip ends. Furthermore, uniaxial tensile tests on the epoxy adhesive and the CFRP strip are used for ...

Mechanical performance of cold-curing epoxy adhesives after different mixing and curing procedures

Abstract This paper presents strength, stiffness, and porosity characteristics of commercially available cold-curing epoxy adhesives for structural engineering applications in the field of externally bonded and/or near-surface mounted composite strip reinforcements. Depending on specific requirements, accelerated curing of the adhesive under high temperatures might be necessary. Experimental investigations aimed at assessing the possible differences in strength and stiffness between samples cured at elevated temperatures for a defined time span and the ones cured at room temperature. It could be demonstrated that for the same specimen age, nominal tensile strength and stiffness are lower after an initial accelerated curing process at elevated temperatures. Furthermore, it could be shown that the specimens after an accelerated curing at elevated temperatures exhibited an increased porosity. The development of a numerical code for image analysis allowed a detailed inspection of several fracture surfaces and subsequently to assess the level of decrease in available cross-section due to an increased overall porosity. Cross-section area losses in the range of 10–15% compared to the reference specimens could be deduced. The subsequent derivation of the actual tensile strength exhibits smaller differences between the room and high temperature exposed specimens while curing. Regardless of the short-term material strength, the observed porosity might be subject of important durability issues on a long-term and needs further investigation.

Prestressed CFRP Strips for Concrete Bridge Girder Retrofitting: Application and Static Loading Test

AbstractThis paper presents an investigation on the practicability and structural efficiency of prestressed carbon-fiber–reinforced polymer (CFRP) strips with a gradient anchorage in the framework of a bridge-strengthening application in Poland. The nonmechanical anchorage system avoids the installation of metallic bolts and plates, with the exception of a temporary support frame. Two 18.4-m-long large-scale prestressed concrete girders were produced following the drawings of the existing bridge construction. One girder served as a reference, and the second one was strengthened with two prestressed CFRP strips. In this case, the initial negative cambering was leveled out by a layer of dry shotcrete. CFRP strips with a prestrain of 0.58% were applied for flexural upgrading. Both girders with a total length of 18.4 m were finally statically loaded up to failure to assess the strengthening efficiency in flexure of the retrofitting technique used. Tensile failure of the CFRP strips was reached, indicating an ...

Structural Strengthening of Concrete with Fiber Reinforced Cementitious Matrix (FRCM) at Ambient and Elevated Temperature - Recent Investigations in Switzerland

This paper presents recent experimental investigations on structural strengthening by means of (Carbon) Fiber Reinforced Cementitious Matrix (FRCM) in Switzerland. A first test series deals with full-scale reinforced concrete slabs strengthened with one or two composite reinforcement meshes embedded in a shotcrete layer. Static load tests up to failure show the efficiency of the strengthening in terms of increased yield and ultimate load compared to the reference specimen. Due to the initially necessary straightening of the textile, the contribution at lower deflection levels is limited. Only with advanced cracking and crack opening, the mesh develops its full contribution. Ultimate load is reached after a prompt relative slip of the mesh in the shotcrete. In the post-peak domain, failure by concrete crushing was observed. To study the residual tensile strength of the carbon reinforcement after exposure to high temperatures, various tensile tests on small rovings previously cut out of a composite mesh were performed. The specimens were heated to temperatures of 300 °C, 500 °C, 700 °C, and 1000 °C, kept at that level for 30 minutes, and finally cooled down to room temperature. The subsequent tensile tests performed at room temperature revealed a significant drop in the residual tensile strength for exposure temperature higher than 300 °C. A final test was performed on a reinforced concrete slab strip strengthened with a shotcrete layer including a composite mesh as tensile reinforcement. Under a constant service load, the slab was exposed to fire with a temperature rise according to a European standard curve (ETK) for two hours. The slab could withstand the applied loads for the full two hours, during which the composite mesh reached a temperature of about 440 °C. This observation is consistent with the results from tensile tests on filaments, clearly indicating a residual tensile strength after exposure at a similar temperature. The temperature in the internal steel reinforcement did not trespass a critical value of 500 °C as proposed by current design recommendations.

Prestressed FRP Systems

This chapter provides an overview on the state-of-the-art in prestressing systems for the structural retrofitting of reinforced concrete (RC) structures using Externally Bonded (EB) Fibre Reinforced Polymers (FRP). Focus is put on flexural strengthening, which currently is the most common application field for composite materials in structural engineering. The manuscript provides information regarding commercially available prestressing systems and their anchorage procedures. In addition to conventional mechanical anchorages, the innovative ‘gradient anchorage’ that lacks any remaining plates or bolts is also presented. Additionally, the authors mention various current prototypes at the laboratory-scale level. Performed experimental investigations, results, and conclusions represent the core content of this chapter. Several studies from various universities and research institutes worldwide are presented and explained. In these research projects, the previously mentioned systems are applied to specific reinforced or prestressed reinforced concrete members for strengthening purposes. Static and/or dynamic loading indicate the efficiency of the retrofitting concept compared to the reference structure. Generally, prestressed FRP will be demonstrated to follow the principle of conventional prestressed concrete by resulting in higher cracking, yielding, and bearing loads. Especially under service loads, the structural behaviour is improved. A special section is dedicated to prestressed near-surface-mounted (NSM) systems. In addition to the experiments section, calculation techniques for designing prestressed FRP for flexural strengthening are also handled. In shear strengthening and column confinement, prestressed FRP has been limited to notably few research applications to date. Nonetheless, an overview is given and future possible employment is discussed. Eventually, examples from real structural retrofitting projects should provide the practitioners with some background to better disseminate the retrofitting technique in question. The concluding section summarizes the actual situation and identifies needs for future research.

Development of Rolling Technology for an Iron-Based Shape-Memory-Alloy

Low cost Fe-Mn-Si based shape memory alloys (SMA) have drawn much attention during the last two decades as a cost-effective alternative to the expensive Ni-Ti based SMA. In particular, the alloy Fe-17Mn-5Si-10Cr-4Ni-1(V,C) (mass%), which has been developed at Empa shows very promising properties with regard to potential commercial applications in civil and mechanical engineering. This alloy has a higher reverse transformation temperature and larger thermal hysteresis in comparison to the Ni-Ti based alloys, which is adequate for producing stable recovery stresses at room temperature. Furthermore, recovery stresses of up to 300 MPa after heating to only 160°C can be achieved without so-called ‘training’ treatment. Furthermore, the alloy can be easily and cost effectively produced under standard air melting and casting conditions. For availability of these heavily microstructure dependent skills for civil and mechanical engineering, e.g. as prestressing elements in concrete structures or coupling/clamping devices, a process chain for manufacturing is necessary. Therefore, a hot and cold rolling technology for strip production with thermal heat treatment processes was developed at TU Bergakademie on base of experimental simulation results. The last one helps to understand the dependencies of deformation parameters, the deformation behavior and their influence to the microstructure evolution in correlation to the recovery.This paper discusses the basic material properties, recovery stress formation behavior and finally the feasibility of the alloy as reinforcing elements in civil engineering applications by using a rolling technology for flat products.

Calculation Technique for Externally Unbonded CFRP Strips in Structural Concrete Retrofitting

AbstractThis paper presents a calculation procedure for externally unbonded carbon fiber–reinforced polymer (CFRP) strips anchored at their ends in flexural strengthening of existing reinforced concrete structures. Due to strain incompatibility between the composite strip and the neighboring concrete surface, the well-known conventional cross-section analysis (CSA) employed for bonded reinforcements cannot be implemented. An iterative force equilibrium and strip strain adaptation with previously defined constitutive materials laws together with an optimization procedure in a numerical computing environment are used to overcome the computational complexities. For validation purposes, two static loading tests on reinforced concrete beams with an externally prestressed CFRP strip without bond except in the anchorage zones are compared to the numerical predictions. Prior to the final evaluation, a model for the anchorage resistance is implemented in the algorithm in order to capture the ultimate load due to a...

Preliminary study on the influence of fibre orientation in fibre reinforced mortars

The influence of steel fibres in fibre reinforced mortars is considered in the paper. Fresh mortar samples with randomly distributed fibres are placed in a spiral coil and exposed to electromagnetic filed so as to achieve orientation of fibres. The location and orientation of fibres is defined by x-ray images. The bending strength testing with deflection checking using the ICS system (Digital Image Correlation System) is conducted. Considering the fibre orientation and load applied, the results exhibit better energy dissipation and greater energy at fracture in case of samples with oriented fibres.

The Gradient Anchorage Method for Prestressed CFRP Strips: from the Development to the Strengthening of an 18 M Long Bridge Girder

Abstract The external bonding of carbon fiber-reinforced polymer (CFRP) strips by two-component epoxy adhesive on the concrete surfaces of buildings and bridges is a retrofitting method accepted worldwide. The gradient anchorage (GA) is an anchoring method especially developed to anchor prestressed CFRP strips to concrete elements without a need for mechanical clamping after the installation phase. This method takes advantage of the adhesives property to undergo accelerated curing when heated. The results of more than fifteen years of research on the development of the gradient anchorage at the Swiss Federal Laboratories for Materials Science and Technology (Empa) are presented in this paper. The basic principles and application steps are explained, and the main results starting from the development of the technique up to the testing of real scale girders are described, and the new challenges posed by this innovative system are highlighted. The gradient anchorage is a valid alternative to a mechanically anchored system for prestressed FRP (P-FRP).

Behaviour of Prestressed CFRP Anchorages during and after Freeze-Thaw Cycle Exposure

The long-term performance of externally-bonded reinforcements (EBR) on reinforced concrete (RC) structures highly depends on the behavior of constituent materials and their interfaces to various environmental loads, such as temperature and humidity exposure. Although significant efforts have been devoted to understanding the effect of such conditions on the anchorage resistance of unstressed EBR, with or without sustained loading, the effect of a released prestressing has not been thoroughly investigated. For this purpose, a series of experiments has been carried out herein, with concrete blocks strengthened with carbon fiber-reinforced polymer (CFRP) strips, both unstressed, as well as prestressed using the gradient anchorage. The gradient anchorage is a non-mechanical technique to anchor prestressed CFRP by exploiting the accelerated curing property of epoxy under higher temperatures and segment-wise prestress-force releasing. Subsequently, strengthened blocks are transferred into a chamber for exposure in dry freeze-thaw cycles (FTC). Following FTC exposure, the blocks are tested in a conventional lap-shear test setup to determine their residual anchorage resistance and then compared with reference specimens. Blocks were monitored during FTC by conventional and Fabry–Pérot-based fiber optic strain (FOS) sensors and a 3D-digital image correlation (3D-DIC) system during gradient application and lap-shear testing. Results indicate a reduction of residual anchorage resistance, stiffness and deformation capacity of the system after FTC and a change in the failure mode from concrete substrate to epoxy-concrete interface failure. It was further observed that all of these properties experienced a more significant reduction for prestressed specimens. These findings are presented with a complementary finite element model to shed more light onto the durability of such systems.